The statocyst-oculomotor system of cephalopods attains the highest level of complexity among invertebrates, with striking parallels in structure and function with its vertebrate counterpart, the vestibulo-oculomotor system, In cephalopods, research during the past 18 years has concentrated on the system in Octopus. The aim of this project is a detailed morphological and physiological analysis of the decapod (cuttlefish and squid) system, which at the level of the receptor sites more closely parallels the vertebrate system and, contrary to the Octopus system, additionally provides suitable preparations for intracellular recordings from the receptor hair cells, for chronic recordings from the relevant brain areas and chronic measurements of those compensatory eye movements that are controlled by the equilibrium receptor organs. Specific aims of this project and methods (in brackets) for achieving them, are; (a) to analyze the morphometry, ultrastructure, and neuronal and synaptic organization of the receptor epithelia (light and electron microscopy, silver and cobalt staining), (b) to determine the functional characteristics of the receptor cells and afferent units (intra- and extracellular recordings), (c) to investigate the influence of the efferent innervation on the afferent units (electrophysiology), (d) to determine the transmitters of the afferent and efferent fiber system (HPL-chromatography, electrophysiology), (e) to describe the afferent and efferent brain pathways of the statocyst nerves (cobalt staining), (f) to describe the neuroanatomy of oculomotor center (cobalt staining), (g) to portray the arrangement of the extraocular eye muscles and eye muscle nerves (gross- anatomy, silver and cobalt staining), (h) to determine the transmitters of the extraocular eye muscle motoneurons (HPL-chromatography, muscle activity during transmitter application), and (i) to analyze the compensatory eye movements and post-rotatory nystagmus (scleral search coil method). Comparative information on similarities and differences of the cephalopod and vertebrate systems can substantially contribute to our understanding of the basic principles of morphology, physiology and pathology of these systems in higher vertebrates, including man. Since in cephalopods the various parts of the system furthermore are easily accessible for operations, the cephalopod system may even serve in future experimental research as a useful invertebrate model of the vertebrate system.